1. Field of the Invention
The present invention relates generally to dental equipment for treating a root canal and in particular, to a system for automatically detecting any desired position in a root canal while enlarging the same. Such detection is possible even if the root canal is moisturized with liquid medicines.
2. Description of the Related Art
In endodontics, germs and the like are generally found in the root canal. Particularly, when the root canal is infected, germs and the like may enter deeply into the dentin. Both mechanical and chemical means are used to eliminate such germs in the root canal in an effort to prevent further infection. Also, the root canal should be fully closed all the way to its apical foramen so as to completely eliminate germs.
Accordingly, it is essential to accurately detect the position of the end of the root canal, known as the apical foremen, by mechanical means such as by a reamer or a file.
In order to measure the length of the root canal, three different methods have heretofore been used in combination: those methods comprise determining by hand when a file is in contact with the apical foramen, taking an X-ray with a reamer inserted in the root canal, or electrically detecting the apical foramen by means of impedance variations. A mechanical cutter such as a hand-operated reamer or file, or an engine reamer or ultrasonic cutter, is then used to enlarge the root canal. It is, however, difficult even for a skilled dentist to efficiently perform such a series of operations, which are time-consuming and may cause patients to suffer severe pain.
FIG. 5 shows a system designed to overcome the foregoing disadvantages. Specifically, this prior art system includes a mechanical cutter 2 having an electrode 3 in the form of a reamer to be inserted into a root canal 1. Another or counter electrode 5 is placed in contact with an oral membrane 4. The two electrodes 3, 5 are coupled to an electrical means 6 for detecting any given position in the root canal. The electrical means 6 is, in turn, coupled to a relay switch 7. The reamer 3 is used to enlarge the root canal 1 as it is advanced toward the apical foramen 8. The electrical means 6 functions to detect a change in impedance between the two electrodes 3, 5. This change may occur when the reamer 3 reaches the apical foramen 8. The relay switch 7 receives an output signal from the electrical means 6 whereby the mechanical cutter is stopped.
In this prior art system, the electrical measuring means 6 has an alternating power source with a frequency of 200 Hz. When the reamer 3 reaches the apical foramen 8, the value of a voltage is measured. This value is then used as a reference value indicating that the reamer has reached the apical foramen. It is reported, however, that such a value may vary depending on age of patients, type of teeth, or shape of the root canal involved. Thus, accurate measurement or detection can not be expected. Also, the interior of the root canal must be dried when the system is in use. During the enlargement of the root canal, the reamer may be bent or otherwise broken due to frictional contact with the dentin.
There has recently been developed a system for detecting any given position in the root canal by means of impedance differential. Voltages as used have different frequencies, for example, 1 KHz and 5 KHz. Waveforms of those frequencies are superposed to provide a composite or voltage waveform. Impedance of a tooth may vary depending on the two different frequencies. Then, the difference between impedances depending on each of two frequencies is measured at a position in the root canal where a change in impedance is low. When the difference becomes the smallest, it is understood that the reamer has reached the apical foramen. Accurate detection of any given position in the root can thus be effected regardless of the age of the patients, type of teeth or shape of the root canal involved.
Liquid medicines and the like which are used to clean the interior of the root canal are high in electrical conductivity or low in electrical resistivity. The difference suffers a dispersion in the vicinity of the apical foramen where the impedance of the tooth is low due to the presence of liquid medicines and the like. As a result, the prior art system cannot accurately detect the position of the apical foramen in the event that liquid medicines and the like are present in the root canal. Furthermore, the prior art system cannot be operative while the root canal is being enlarged.